Memory devices are typically provided as internal, semiconductor, integrated circuits in apparatuses such as computers or other electronic devices. There are many different types of memory including random-access memory (RAM), read only memory (ROM), dynamic random access memory (DRAM), synchronous dynamic random access memory (SDRAM), and non-volatile (e.g., phase change memory, flash) memory.
Non-volatile memories are useful elements of integrated circuits due to their ability to maintain data after removal of a power supply. Phase change materials have been investigated for use in non-volatile memory cells. Phase change memory (PCM) elements include phase change materials, such as chalcogenic semiconductor materials (e.g., chalcogenide alloys), that are capable of stably transitioning between amorphous and crystalline phases. Each phase exhibits a particular resistance state that distinguishes the logic values of the memory element. Specifically, an amorphous state exhibits a relatively high resistance and a crystalline state exhibits a relatively low resistance. One of different logic levels (e.g., logic 1 or logic 0) can be assigned to each of these states. In the chalcogenides, the resistivity may vary by two or more orders of magnitude when the material passes from the amorphous (more resistive) phase to the crystalline (more conductive) phase, and vice versa.
In one embodiment, each memory cell typically includes both a phase change memory element (PM) connected in series with a select device (SD). The SD may be a switching element that includes a diode, a transistor, or an ovonic threshold switch (OTS). Also, the OTS is constructed with a chalcogenic material (i.e., it is an alloy containing at least one chalcogenic element). The OTS begins to conduct when a voltage above a threshold voltage Vth is applied across the switch.
The threshold voltage Vth of the OTS is subject to drift over time. The threshold voltage drift may be harmful for OTS-selected memory arrays since it may prevent the storage element of chalcogenic material from being correctly read. For example, if the threshold voltage Vth of the selector is not known with satisfactory precision and the chalcogenic storage element is crystalline (e.g., stores a logic “1”), the memory cell may be read as a logic “0” since, at the reading voltage, the switch has not yet transitioned to the conductive state.